Laser techniques have been developed which make it convenient to obtain various fundamental frequencies of coherent laser light by utilizing solid, gas, and liquid media. Outstanding characteristics of these solid-state lasers include being small, inexpensive, and requiring no maintenance; their output, however, is limited to the near-infrared region of the spectrum and is of low power. In many applications, laser light having frequencies above those conveniently obtainable is required. Nonlinear optical crystals have, therefore, frequently been employed to convert coherent light of a fundamental frequency into light with a frequency twice the fundamental frequency. This conversion is termed "second harmonic generation" (SHG).
The possibility of using organic molecules in SHG and other nonlinear optical devices has generated much interest recently. Certain substituted aromatic molecules are known to exhibit large optical nonlinearities. Application potential of organic material in nonlinear optical devices is greater than that of conventional inorganic electrooptic materials because of bandwidth limitations of the latter materials.
European Patent Application EP No. 0 218 938 A2 discloses polymeric nonlinear optical materials and devices. Optically nonlinear media are obtained by incorporating an optically nonlinear organic guest molecule (NLO molecule( in an appropriate polymeric host matrix, followed by poling to provide a directional orientation of the guest molecule in the polymeric host. Disclosed hosts are glassy polymers including polyacrylates, polymethacrylates, clear epoxies, polystyrene, and polycarbonates. A disadvantage of this approach is that the guest NLO molecule displays only limited solubility, generally less than about 20% by weight, in the host polymer, thus limiting the magnitude of the nonlinear optical response achievable.
U.S. Pat. No. 4,720,355 discloses poly(N,N-dialkylacrylamides) as host polymers. These polymeric host materials are better solvents for the guest NLO molecules, thus providing more homogeneous mixtures and allowing higher loadings of the NLO guest. However, it is desirable to be able to incorporate even higher loadings of NLO molecules into polymeric materials for the preparation of optically nonlinear media.
An additional disadvantage of the host-guest approach is that a small NLO molecule is relatively free to rotate and diffuse within the polymeric host. In some instances diffusion of the guest out of the host polymer may occur.
U.S. Pat. No 4,694,066 discloses thermotropic liquid crystalline polymers having a comb structure of mesogenic side chains wherein the mesogens also exhibit second order nonlinear susceptibility. These polymers, in which the NLO molecule is covalently bound as a side chain to the polymer backbone, are obtained by the steps of: (1) synthesizing a mesogenic NLO molecule, (2) preparing a derivative of that molecule which contains a polymerizable group, and (3) polymerizing the resultant NLO monomer from step 2 to produce the desired polymer. A disadvantage of this approach is that preparation of the NLO monomer generally involves a multi-step, difficult, and expensive synthetic sequence. An additional and even more serious disadvantage of this approach is that many contemplated NLO monomers have functional groups which would be incompatible with the desired polymerization reaction. For example, many of the donor and acceptor groups which lead to large nonlinear optical susceptibilities can function as chain transfer agents (thus interfering with polymerization) or as retarders or inhibitors of free radical polymerizations.
U.S. Pat. No. 4,694,048 teaches reaction of a cyclic anhydride-containing polymer with an aromatic hydrazine. The product is then cyclodehydrated to produce a thermoplastic polymer containing recurring cyclic hydrazide groups, the polymer being useful as a nonlinear optical substrate. A disadvantage of the teachings in U S. Pat. No. 4,694,048, however, is limited availability of anhydride-containing polymers or copolymers and also the generally low molecular weights of those polymers which are available.
Once an optically nonlinear medium has been obtained by a poling process, rotational and/or diffusional motion may have detrimental effects upon the directional orientation of the NLO molecule. This can lead to a decay in, and eventually to a disappearance of, the nonlinear optical response of the medium.